The present invention concerns a voltage converter of the flux converter type. More particularly, it relates to a flux converter type voltage converter having a self-regulating synchronous rectifier in the secondary circuit.
These types of voltage converters have a synchronous rectifier in the secondary circuit which is typically realized with MOSFETs to attain the most favorable possible efficiency. Numerous topologies are known from the prior art, such as single-ended, balanced, half-bridge, push-pull, full-bridge, or phase shift flux converters.
In particular, however, the principal of self-regulation of synchronous rectifiers, namely the usage of voltage traces already present and/or occurring in the converter circuit for switching on and/or off the active switching elements in synchronization with the switches in the primary circuit, can be problematic in principle in many flux converter topologies, particularly those which are not single-ended flux converters: when, in a balanced, half-bridge, full-bridge, or phase shift flux converter, all of the semiconductors are in a switched-off state, the freewheeling current must be conducted at this time in the secondary circuit through the active (rectifier) switching elements, and, if these elements are realized with MOSFETs, an appropriate drive signal must be present. However, this cannot be directly generated from the transformer, for example by means of an auxiliary winding provided for the drive signal, without any further means.
For these reasons, the principle of self-regulation is known in principle, but for this reason and other problems connected with topology, it is disadvantageous precisely in flux converters of the topologies mentioned.
Rather, in these flux converter topologies (balanced, half-bridge, full-bridge, or phase shift flux converters) or other topologies, driving of power semiconductors of a synchronous rectifier in the secondary circuit, as it is described in an analogous way in the German utility model 299 01 322, is achieved by means of external control, as is schematically described in FIG. 6 for the prior art: a PWM control unit 10 hereby controls both an (otherwise known) switching arrangement in the primary circuit itself, and, via a control unit 14 located by a transformer 16 in the secondary circuit, a pair of powered semiconductors 18, 20 acting as a synchronous rectifier, with the converter output voltage signal dropping off over a load resistor RL.
The voltage curves of FIG. 7 illustrate the switching behavior of the rectifier elements 18 and 20 relative to the transformer voltage (first curve), which, conditioned by the control in the primary circuit, oscillates in the way shown around the zero voltage: in contrast, the drive signals for the transistors 18, 20 (second and third curve) have a level greater than zero for activation of the respective semiconductor for the freewheeling current.
In any case, as is already obvious with reference to FIG. 6, this type of external control of the active switching elements of the synchronous rectifier in the secondary circuit is expensive.
One object of the present invention is therefore to simplify the driving of active switching elements of a synchronous rectifier in a voltage converter of this type having the topologies mentioned, in particular to reduce the expense for circuit technology, with the principle of self-regulation, i.e., regeneration of the control signals from signals already present in the converter circuit, without the necessity for external signal logics being used.
Thus, in accordance with one form of the invention, a capacitor device is provided which temporarily stores the driving energy (and/or drive voltage) for the active switching elements in its charge in such a way that, particularly at those times in the operating cycle at which no voltage signal is applied to the auxiliary winding according to the invention in the secondary circuit, the driving operation and therefore the regular mode of operation of the synchronous rectifier can be ensured.
In an advantageous way according one form of the invention, usage is thereby made of the circumstance that the signal generated by the auxiliary winding is synchronous with the transformer voltage, so that the switching on and/or off of the active switching elements of the rectifier occurs with high precision and thereby with low loss. The semiconductor element, diode, or transistor used according to the invention for driving the synchronous rectifier (to be precise: the control terminal of an appropriate rectifier switching element), thereby allows, in a way which requires extremely simple circuit technology, signal generation and signal application through the cooperation of auxiliary winding and capacitor.
It is preferred that a capacitance value for the capacitor device be selected which is significantly higher than a driving capacitance (e.g., gate capacitance in the case of a MOSFET) of the control terminal, so that the charging behaviors concerned ensure reliable driving and stable switching. Selecting a capacitance of the capacitor device which is at least five to ten times the driving capacitance for the active circuit element has hereby particularly proven itself.
In principle, the present invention is suitable for any desired topology in the primary and secondary circuits; in the primary circuit, balanced, full-bridge, half-bridge, or phase shift topologies are to be considered particularly preferred, and, in the secondary circuit, a current doubler or the simple middle point configuration with a storage inductor.
In a preferred embodiment of the invention, the secondary circuit is realized with two circuit arms as a bridge rectifier, so that, for each circuit arm, an active switching element is present which is supplied with charge and/or voltage from an associated auxiliary winding of the pair of auxiliary windings. It can hereby be favorable, on one hand, to provide a shared capacitor as a capacitor device for both arms; alternatively, a further preferred embodiment of the invention provides a series circuit made of inductor (i.e., respective auxiliary winding) and capacitor for each arm, with, in this case, the semiconductor element being realized as a transistor (particularly preferred: MOSFET) and its control signal being received from a node between inductor and capacitor of a respective opposing arm.
The advantage relative to an embodiment with diodes as the semiconductor element and/or only one capacitor is that, through this type of circuit, the capacitor is both charged and discharged by the associated auxiliary winding and the control terminals of the active switching elements are actively drawn to a zero level during the switching-off phases provided, so that, in particular during rapid changes in voltage, capacitive effects of the power semiconductor itself cannot lead to unintended switching on.
A further preferred embodiment of the invention is to combine each of the arms with a voltage limiter because the voltage over a capacitor used as a capacitor element in each arm depends on the input voltage of the converter and therefore, for example during large variations in input voltage, a maximum drive voltage can be exceeded at the active switching element. For this purpose, it particularly suggests itself that suitably located and driven MOSFETs be connected upstream on the channel side from the respective control terminals of the active switching elements for voltage limiting.
As a result, through the present invention, a voltage converter with a self-regulated synchronous rectifier arises, having surprisingly simple circuit technology, which is distinguished by very precise switching behavior of the rectifier elements in the secondary circuit and thereby low loss. Simultaneously, the low number of circuit elements used minimizes the expense for production technology, so that the present invention has great advantages, particularly from the viewpoint of production technology.